Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in 3GPP, TR 25.913 (“Requirements for Evolved UTRA and Evolved UTRAN”, www.3gpp.org). In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed, including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in Rel. 8 LTE.
E-UTRAN Architecture
The overall architecture is shown in FIG. 1, and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of one or more eNodeBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (UL QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (S-GW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state UEs, the S-GW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g., parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the S-GW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the UE to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE
The downlink component carrier of a 3GPP LTE system is subdivided in the time-frequency domain in so-called subframes. In 3GPP LTE each subframe is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each subframe consists of a given number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consist of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g., employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 4. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.9.0 or 9.0.0, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
The term “component carrier” refers to a combination of several resource blocks. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Further Advancements for LTE (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radio communication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved in the 3GPP. The study item covers technology components to be considered for the evolution of E-UTRA, e.g., to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers (CCs) are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz, even though these cells in LTE are in different frequency bands. A UE may simultaneously receive or transmit on one or multiple CCs depending on its capabilities:                A Rel-10 UE with reception and/or transmission capabilities for CA can simultaneously receive and/or transmit on multiple CCs corresponding to multiple serving cells;        A Rel-8/9 UE can receive on a single CC and transmit on a single CC corresponding to one serving cell only.        
Carrier aggregation (CA) is supported for both contiguous and non-contiguous CCs with each CC limited to a maximum of 110 Resource Blocks in the frequency domain using the Rel-8/9 numerology.
It is possible to configure a UE to aggregate a different number of CCs originating from the same eNB and of possibly different bandwidths in the UL and the DL.
It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may not be possible to configure a mobile terminal with more uplink component carriers than downlink component carriers.
In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not provide the same coverage.
Component carriers shall be LTE Rel-8/9 compatible. Nevertheless, existing mechanisms (e.g., barring) may be used to avoid Rel-8/9 UEs to camp on a component carrier.
The spacing between center frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively. The transport channels are described between MAC and Layer 1, the logical channels are described between MAC and RLC.
When carrier aggregation (CA) is configured, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the NAS mobility information (e.g., TAI), and at RRC connection re-establishment/handover, one serving cell provides the security input. This cell is referred to as the Primary Cell (PCell). In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC), while in the uplink it is the Uplink Primary Component Carrier (UL PCC).
Depending on UE capabilities, Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells. In the downlink, the carrier corresponding to an SCell is a Downlink Secondary Component Carrier (DL SCC), while in the uplink it is an Uplink Secondary Component Carrier (UL SCC).
The configured set of serving cells for a UE therefore always consists of one PCell and one or more SCells:                For each SCell the usage of uplink resources by the UE in addition to the downlink ones is configurable (the number of DL SCCs configured is therefore always larger or equal to the number of UL SCCs and no SCell can be configured for usage of uplink resources only);        From a UE viewpoint, each uplink resource only belongs to one serving cell;        The number of serving cells that can be configured depends on the aggregation capability of the UE;        PCell can only be changed with handover procedure (i.e., with security key change and RACH procedure);        PCell is used for transmission of PUCCH;        Unlike SCells, PCell cannot be de-activated;        Re-establishment is triggered when the PCell experiences Rayleigh fading (RLF), not when SCells experience RLF;        Non-access stratum (NAS) information is taken from the downlink PCell.        
The configuration and reconfiguration of component carriers can be performed by RRC. Activation and deactivation is done via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. The reconfiguration, addition and removal of SCells can be performed by RRC. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage with the target PCell. When adding a new SCell, dedicated RRC signalling is used for sending all required system information of the SCell, i.e., while in connected mode, UEs need not acquire broadcasted system information directly from the SCells.
When a user equipment is configured with carrier aggregation there is one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as “DL anchor carrier”. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously, but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats, called CIF.
A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carrier does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.                LTE RRC States        
The following is mainly describing the two main states in LTE: “RRC_IDLE” and “RRC_CONNECTED”.
In RRC_IDLE the radio is not active, but an ID is assigned and tracked by the network. More specifically, a mobile terminal in RRC_IDLE performs cell selection and reselection—in other words, it decides on which cell to camp. The cell (re)selection process takes into account the priority of each applicable frequency of each applicable Radio Access Technology (RAT), the radio link quality and the cell status (i.e., whether a cell is barred or reserved). An RRC_IDLE mobile terminal monitors a paging channel to detect incoming calls, and also acquires system information. The system information mainly consists of parameters by which the network (E-UTRAN) can control the cell (re)selection process. RRC specifies the control signalling applicable for a mobile terminal in RRC_IDLE, namely paging and system information. The mobile terminal behavior in RRC_IDLE is specified in TR 25.912, e.g., Chapter 8.4.2 incorporate herein by reference.
In RRC_CONNECTED the mobile terminal has an active radio operation with contexts in the eNodeB. The E-UTRAN allocates radio resources to the mobile terminal to facilitate the transfer of (unicast) data via shared data channels. To support this operation, the mobile terminal monitors an associated control channel which is used to indicate the dynamic allocation of the shared transmission resources in time and frequency. The mobile terminal provides the network with reports of its buffer status and of the downlink channel quality, as well as neighboring cell measurement information to enable E-UTRAN to select the most appropriate cell for the mobile terminal. These measurement reports include cells using other frequencies or RATs. The UE also receives system information, consisting mainly of information required to use the transmission channels. To extend its battery lifetime, a UE in RRC_CONNECTED may be configured with a Discontinuous Reception (DRX) cycle. RRC is the protocol by which the E-UTRAN controls the UE behavior in RRC_CONNECTED.                Logical and Transport Channels        
The MAC layer provides a data transfer service for the RLC layer through logical channels. Logical channels are either Control Logical Channels which carry control data such as RRC signalling, or Traffic Logical Channels which carry user plane data. Broadcast Control Channel (BCCH), Paging Control channel (PCCH), Common Control Channel (CCCH), Multicast Control Channel (MCCH) and Dedicated Control Channel (DCCH) are Control Logical Channels. Dedicated Traffic channel (DTCH) and Multicast Traffic Channel (MTCH) are Traffic Logical Channels.
Data from the MAC layer is exchanged with the physical layer through Transport Channels. Data is multiplexed into transport channels depending on how it is transmitted over the air. Transport channels are classified as downlink or uplink as follows. Broadcast Channel (BCH), Downlink Shared Channel (DL-SCH), Paging Channel (PCH) and Multicast Channel (MCH) are downlink transport channels, whereas the Uplink Shared Channel (UL-SCH) and the Random Access Channel (RACH) are uplink transport channels.
A multiplexing is then performed between logical channels and transport channels in the downlink and uplink respectively.
Layer 1/Layer 2 (L1/L2) Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other data-related information (e.g., HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a subframe, assuming that the user allocation can change from subframe to subframe. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the subframes. The TTI length may be fixed in a service area for all users, may be different for different users, or may even be dynamic for each user. Generally, the L1/L2 control signaling needs only be transmitted once per TTI.
The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). A PDCCH carries a message as a Downlink Control Information (DCI), which includes resource assignments and other control information for a mobile terminal or groups of UEs. In general, several PDCCHs can be transmitted in one subframe.
It should be noted that in 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH.
With respect to scheduling grants, the information sent on the L1/L2 control signaling may be separated into the following two categories, Shared Control Information (SCI) carrying Cat 1 information and Downlink Control Information (DCI) carrying Cat 2/3 information.
Shared Control Information (SCI) Carrying Cat 1 Information
The shared control information part of the L1/L2 control signaling contains information related to the resource allocation (indication). The shared control information typically contains the following information:                A user identity indicating the user(s) that is/are allocated the resources.        RB allocation information for indicating the resources (Resource Blocks (RBs)) on which a user(s) is/are allocated. The number of allocated resource blocks can be dynamic.        The duration of assignment (optional), if an assignment over multiple subframes (or TTIs) is possible.        
Depending on the setup of other channels and the setup of the Downlink Control Information (DCI)—see below—the shared control information may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Downlink Control Information (DCI) Carrying Cat 2/3 Information
The downlink control information part of the L1/L2 control signaling contains information related to the transmission format (Cat 2 information) of the data transmitted to a scheduled user indicated by the Cat 1 information. Moreover, in case of using (Hybrid) ARQ as a retransmission protocol, the Cat 2 information carries HARQ (Cat 3) information. The downlink control information needs only to be decoded by the user scheduled according to Cat 1. The downlink control information typically contains information on:                Cat 2 information: Modulation scheme, transport-block (payload) size or coding rate, MIMO (Multiple Input Multiple Output)-related information, etc. Either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated resource blocks)        Cat 3 information: HARQ related information, e.g., hybrid ARQ process number, redundancy version, retransmission sequence number        
Downlink control information occurs in several formats that differ in overall size and also in the information contained in its fields. The different DCI formats that are currently defined for LTE are as follows and described in detail in 3GPP TS 36.212, “Multiplexing and channel coding”, section 5.3.3.1 (available at http://www.3gpp.org and incorporated herein by reference).
Format 0: DCI Format 0 is used for the transmission of resource grants for the PUSCH.
For further information regarding the DCI formats and the particular information that is transmitted in the DCI, please refer to the technical standard or to LTE—The UMTS Long Term Evolution—From Theory to Practice, Edited by Stefania Sesia, Issam Toufik, Matthew Baker, Chapter 9.3, incorporated herein by reference.
Downlink & Uplink Data Transmission
Regarding downlink data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH), along with the downlink packet data transmission. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the data is transmitted (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the mobile terminal (receiver) to identify the resources on which the data is transmitted.        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling, this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier (“cross-carrier scheduling”). This other, cross-scheduled component carrier could be for example a PDCCH-less component carrier, i.e., the cross-scheduled component carrier does not carry any L1/L2 control signaling.        The Transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g., the number of resource blocks assigned to the user equipment)) allows the user equipment (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. The modulation scheme may be signaled explicitly.        Hybrid ARQ (HARQ) Information:                    HARQ process number: Allows the user equipment to identify the hybrid ARQ process on which the data is mapped.            Sequence number or new data indicator (NDI): Allows the user equipment to identify if the transmission is a new packet or a retransmitted packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a PDU prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation).                        UE Identity (UE ID): Tells for which user equipment the L1/L2 control signaling is intended. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the user equipment about the transmission details. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the user equipment should transmit the data (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling, this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier. This other, cross-scheduled component carrier may be for example a PDCCH-less component carrier, i.e., the cross-scheduled component carrier does not carry any L1/L2 control signaling.        L1/L2 control signaling for uplink grants is sent on the DL component carrier that is linked with the uplink component carrier or on one of the several DL component carriers, if several DL component carriers link to the same UL component carrier.        The Transport Format, the user equipment that should be used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g., the number of resource blocks assigned to the user equipment)) allows the user equipment (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.        Hybrid ARQ Information:                    HARQ Process number: Tells the user equipment from which hybrid ARQ process it should pick the data.            Sequence number or new data indicator: Tells the user equipment to transmit a new packet or to retransmit a packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a protocol data unit (PDU) prior to decoding.            Redundancy and/or constellation version: Tells the user equipment which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).                        UE Identity (UE ID): Tells which user equipment should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
There are several different possibilities of how to exactly transmit the information pieces mentioned above in uplink and downlink data transmission. Moreover, in uplink and downlink, the L1/L2 control information may also contain additional information or may omit some of the information. For example:                HARQ process number may not be needed, i.e., is not signaled, in case of a synchronous HARQ protocol.        A redundancy and/or constellation version may not be needed, and thus not signaled, if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre-defined.        Power control information may be additionally included in the control signaling.        MIMO related control information, such as, e.g., pre-coding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.        
For uplink resource assignments (on the Physical Uplink Shared Channel (PUSCH)) signaled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore, it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e., the RV info is embedded in the transport format (TF) field. The Transport Format (TF) respectively modulation and coding scheme (MCS) field has for example a size of 5 bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating redundancy versions (RVs) 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RV0. The size of the CRC field of the PDCCH is 16 bits.
For downlink assignments (PDSCH) signaled on PDCCH in LTE the Redundancy Version (RV) is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signaled on PDCCH. 3 of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries, modulation order and Transport block size info are signaled.                Uplink Access Scheme for LTE        
For Uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single-carrier transmission combined with FDMA with dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR), compared to multi-carrier signals (OFDMA), and the corresponding improved power-amplifier efficiency and assumed improved coverage (higher data rates for a given terminal peak power). During each time interval, Node B assigns users a unique time/frequency resource for transmitting user data, thereby ensuring intra-cell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra-cell interference. Interference due to multipath propagation is handled at the base station (Node B), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one time interval, e.g., a subframe of 0.5 ms, onto which coded information bits are mapped. It should be noted that a subframe, also referred to as transmission time interval (TTI), is the smallest time interval for user data transmission. It is however possible to assign a frequency resource BWgrant over a longer time period than one TTI to a user by concatenation of subframes.                Uplink Scheduling Scheme for LTE        
The uplink scheme allows for both scheduled access, i.e., controlled by eNB, and contention-based access.
In case of scheduled access, the UE is allocated a certain frequency resource for a certain time (i.e., a time/frequency resource) for uplink data transmission. However, some time/frequency resources can be allocated for contention-based access; within these time/frequency resources, UEs can transmit without first being scheduled. One scenario where UE is making a contention-based access is for example the random access, i.e., when UE is performing initial access to a cell or for requesting uplink resources.
For the scheduled access the Node B scheduler assigns a user a unique frequency/time resource for uplink data transmission. More specifically the scheduler determines                which UE(s) that is (are) allowed to transmit,        which physical channel resources (frequency),        Transport format (Modulation Coding Scheme (MCS)) to be used by the mobile terminal for transmission.        
The allocation information is signaled to the UE via a scheduling grant, sent on the L1/L2 control channel. For simplicity reasons this channel may be called uplink grant channel in the following. A scheduling grant message contains at least information which part of the frequency band the UE is allowed to use, the validity period of the grant and the transport format the UE has to use for the upcoming uplink transmission. The shortest validity period is one subframe. Additional information may also be included in the grant message, depending on the selected scheme. Only “per UE” grants are used to grant the right to transmit on the UL-SCH (i.e., there are no “per UE per RB” grants). Therefore, the UE needs to distribute the allocated resources among the radio bearers according to some rules. Unlike in HSUPA there is no UE-based transport format selection. The eNB decides the transport format based on some information, e.g., reported scheduling information and QoS info, and UE has to follow the selected transport format. In HSUPA the Node B assigns the maximum uplink resource, and the UE selects accordingly the actual transport format for the data transmissions.
Since the scheduling of radio resources is the most important function in a shared channel access network for determining Quality of service, there are a number of requirements that should be fulfilled by the UL scheduling scheme for LTE in order to allow for an efficient QoS management.                Starvation of low priority services should be avoided;        Clear QoS differentiation for radio bearers/services should be supported by the scheduling scheme;        The UL reporting should allow fine granular buffer status reports (e.g., per radio bearer or per radio bearer group) in order to allow the eNB scheduler to identify for which Radio Bearer/service data is to be sent;        It should be possible to make clear QoS differentiation between services of different users;        It should be possible to provide a minimum bit rate per radio bearer.        
As can be seen from the above list, one essential aspect of the LTE scheduling scheme is to provide mechanisms with which the operator can control the partitioning of its aggregated cell capacity between the radio bearers of the different QoS classes. The QoS class of a radio bearer is identified by the QoS profile of the corresponding SAE bearer signalled from AGW to eNB as described before. An operator can then allocate a certain amount of its aggregated cell capacity to the aggregated traffic associated with radio bearers of a certain QoS class. The main goal of employing this class-based approach is to be able to differentiate the treatment of packets depending on the QoS class they belong to.                Buffer Status Reporting/Scheduling Request Procedure for Uplink Scheduling        
The usual mode of scheduling is dynamic scheduling, by means of downlink assignment messages for the allocation of downlink transmission resources and uplink grant messages for the allocation of uplink transmission resources; these are usually valid for specific single subframes. They are transmitted on the PDCCH using C-RNTI of the UE as already mentioned before. Dynamic scheduling is efficient for services types, in which the traffic is bursty and dynamic in rate, such as TCP.
In addition to the dynamic scheduling, a persistent scheduling is defined, which enables radio resources to be semi-statically configured and allocated to a UE for a longer time period than one subframe, thus avoiding the need for specific downlink assignment messages or uplink grant messages over the PDCCH for each subframe. Persistent scheduling is useful for services such as VoIP for which the data packets are small, periodic and semi-static in size. Thus, the overhead of the PDCCH is significantly reduced compared to the case of dynamic scheduling.
Buffer status reports (BSR) from the UE to the eNodeB are used to assist the eNodeB in allocating uplink resources, i.e., uplink scheduling. For the downlink case, the eNB scheduler is obviously aware of the amount of data to be delivered to each UE; however, for the uplink direction, since scheduling decisions are done at the eNB and the buffer for the data is in the UE, BSRs have to be sent from the UE to the eNB in order to indicate the amount of data that needs to be transmitted over the UL-SCH.
There are basically two types of Buffer Status Report MAC control elements (BSR) defined for LTE: a long BSR (with four buffer size fields corresponding to LCG IDs #0-3) or a short BSR (with one LCG ID field and one corresponding buffer size field). The buffer size field indicates the total amount of data available across all logical channels of a logical channel group, and is indicated in number of bytes encoded as an index of different buffer size levels (see also 3GPP TS 36.321 v 10.5.0 Chapter 6.1.3.1, incorporated herewith by reference). In addition, there is a further type of Buffer Status Report, for use of truncated data, where the Buffer Status Report is 2 bytes long.
Which one of either the short or the long BSR is transmitted by the UE depends on the available transmission resources in a transport block, on how many groups of logical channels have non-empty buffers, and on whether a specific event is triggered at the UE. The long BSR reports the amount of data for four logical channel groups, whereas the short BSR indicates the amount of data buffered for only the highest logical channel group.
The reason for introducing the logical channel group concept is that even though the UE may have more than four logical channels configured, reporting the buffer status for each individual logical channel would cause too much signaling overhead. Therefore, the eNB assigns each logical channel to a logical channel group; preferably, logical channels with same/similar QoS requirements should be allocated within the same logical channel group.
A BSR may be triggered, as an example, for the following events:                Whenever data arrives for a logical channel, which has a higher priority than the logical channels whose buffer are non-empty;        Whenever data becomes available for any logical channel, when there was previously no data available for transmission;        Whenever the retransmission BSR time expires;        Whenever periodic BSR reporting is due, i.e., periodic BSR timer expires;        Whenever there is a spare space in a transport block which can accommodate a BSR.        
In order to be robust against transmission failures, there is a BSR retransmission mechanism defined for LTE; the retransmission BSR timer is started or restarted whenever an uplink grant is restarted. If no uplink grant is received before the retransmission BSR timer expires, another BSR is triggered by the UE.
If the UE has no uplink resources allocated for including a BSR in the transport block (TB) when a BSR is triggered the UE sends a scheduling request (SR) on the Physical Uplink Control Channel (PUCCH), if configured. For the case that there are no D-SR (dedicated Scheduling request) resources on PUCCH configured, the UE will start the Random Access Procedure (RACH procedure) in order to request UL-SCH resources for transmission the BSR info to eNB. However it should be noted that the UE will not trigger SR transmission for the case that a periodic BSR is to be transmitted.
Furthermore an enhancement to the SR transmission has been introduced for a specific scheduling mode where resources are persistently allocated with a defined periodicity in order to save L1/L2 control signalling overhead for transmission grants, which is referred to as semi-persistent scheduling (SPS). One example for a service, which has been mainly considered for semi-persistent scheduling, is VoIP. Every 20 ms a VoIP packet is generated at the Codec during a talk-spurt. Therefore eNB can allocate uplink or respectively downlink resource persistently every 20 ms, which could be then used for the transmission of VoIP packets. In general SPS is beneficial for services with predictable traffic behavior, i.e., constant bit rate, packet arrival time is periodic. For the case that SPS is configured for the uplink direction, the eNB can turn off SR triggering/transmission for certain configured logical channels, i.e., BSR triggering due to data arrival on those specific configured logical channels will not trigger an SR. The motivation for such kind of enhancements is that reporting an SR for those logical channels which will use the semi-persistently allocated resources (logical channels which carry VoIP packets) is of no value for eNB scheduling, and hence should be avoided.
More detailed information with regard to BSR and in particular the triggering of same is explained in 3GPP TS 36.321 V10.5 in Chapter 5.4.5 incorporated herewith by reference.                Logical Channel Prioritization        
The UE has an uplink rate control function which manages the sharing of uplink resources between radio bearers. This uplink rate control function is also referred to as logical channel prioritization procedure in the following. The Logical Channel Prioritization (LCP) procedure is applied when a new transmission is performed, i.e., a Transport block needs to be generated. One proposal for assigning capacity has been to assign resources to each bearer, in priority order, until each has received an allocation equivalent to the minimum data rate for that bearer, after which any additional capacity is assigned to bearers in, for example, priority order.
As will become evident from the description of the LCP procedure given below, the implementation of the LCP procedure residing in the UE is based on the token bucket model, which is well known in the IP world. The basic functionality of this model is as follows. Periodically at a given rate, a token, which represents the right to transmit a quantity of data, is added to the bucket. When the UE is granted resources, it is allowed to transmit data up to the amount represented by the number of tokens in the bucket. When transmitting data the UE removes the number of tokens equivalent to the quantity of transmitted data. In case the bucket is full, any further tokens are discarded. For the addition of tokens it could be assumed that the period of the repetition of this process would be every TTI, but it could be easily lengthened such that a token is only added every second. Basically instead of every 1 ms a token is added to the bucket, 1000 tokens could be added every second. In the following, the logical channel prioritization procedure which is used in Rel-8 is described.
More detailed information with regard to the LCP procedure is explained in 3GPP TS 36.321 V8 in Chapter 5.4.3.1, incorporated herewith by reference.
RRC controls the scheduling of uplink data by signalling for each logical channel: priority where an increasing priority value indicates a lower priority level, prioritisedBitRate which sets the Prioritized Bit Rate (PBR), and bucketSizeDuration which sets the Bucket Size Duration (BSD). The idea behind prioritized bit rate is to support for each bearer, including low priority non-GBR bearers, a minimum bit rate in order to avoid a potential starvation. Each bearer should at least get enough resources in order to achieve the prioritized bit rate (PRB).
The UE shall maintain a variable Bj for each logical channel j. Bj shall be initialized to zero when the related logical channel is established, and incremented by the product PBR×TTI duration for each TTI, where PBR is Prioritized Bit Rate of logical channel j. However, the value of Bj can never exceed the bucket size, and if the value of Bj is larger than the bucket size of logical channel j, it shall be set to the bucket size. The bucket size of a logical channel is equal to PBR×BSD, where PBR and BSD are configured by upper layers.
The UE shall perform the following Logical Channel Prioritization procedure when a new transmission is performed:                The UE shall allocate resources to the logical channels in the following steps:        Step 1: All the logical channels with Bj>0 are allocated resources in a decreasing priority order. If the PBR of a radio bearer is set to “infinity”, the UE shall allocate resources for all the data that is available for transmission on the radio bearer before meeting the PBR of the lower priority radio bearer(s);        Step 2: the UE shall decrement Bj by the total size of MAC SDUs served to logical channel j in Step 1;        
It has to be noted at this point that the value of Bj can be negative.                Step 3: if any resources remain, all the logical channels are served in a strict decreasing priority order (regardless of the value of Bj) until either the data for that logical channel or the UL grant is exhausted, whichever comes first. Logical channels configured with equal priority should be served equally.        The UE shall also follow the rules below during the scheduling procedures above:        the UE should not segment an RLC SDU (or partially transmitted SDU or retransmitted RLC PDU) if the whole SDU (or partially transmitted SDU or retransmitted RLC PDU) fits into the remaining resources;        if the UE segments an RLC SDU from the logical channel, it shall maximize the size of the segment to fill the grant as much as possible;        UE should maximize the transmission of data.        
For the Logical Channel Prioritization procedure, the UE shall take into account the following relative priority in decreasing order:                MAC control element for C-RNTI or data from UL-CCCH;        MAC control element for BSR, with exception of BSR included for padding;        MAC control element for PHR;        data from any Logical Channel, except data from UL-CCCH;        MAC control element for BSR included for padding.        
For the case of carrier aggregation, which is described in a later section, when the UE is requested to transmit multiple MAC PDUs in one TTI, steps 1 to 3 and the associated rules may be applied either to each grant independently or to the sum of the capacities of the grants. Also the order in which the grants are processed is left up to UE implementation. It is up to the UE implementation to decide in which MAC PDU a MAC control element is included when UE is requested to transmit multiple MAC PDUs in one TTI.                Uplink Power Control        
Uplink transmission power control in a mobile communication system serves an important purpose: it balances the need for sufficient transmitted energy per bit to achieve the required Quality-of-Service (QoS), against the needs to minimize interference to other users of the system and to maximize the battery life of the mobile terminal. In achieving this purpose, the role of the Power Control (PC) becomes decisive to provide the required SINR, while controlling at the same time the interference caused to neighboring cells. The idea of classic PC schemes in uplink is that all users are received with the same SINR, which is known as full compensation. As an alternative, 3GPP has adopted for LTE the use of Fractional Power Control (FPC). This new functionality makes users with a higher path loss operate at a lower SINR requirement so that they will more likely generate less interference to neighboring cells.
Detailed power control formulae are specified in LTE for the Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Channel (PUCCH) and the Sounding Reference Signals (SRSs) (section 5.1 in TS 36.213). The formula for each of these uplink signals follows the same basic principles; in all cases they can be considered as a summation of two main terms: a basic open-loop operating point derived from static or semi-static parameters signalled by the eNodeB, and a dynamic offset updated from subframe to subframe.
The basic open-loop operating point for the transmit power per resource block depends on a number of factors including the inter-cell interference and cell load. It can be further broken down into two components, a semi-static base level P0, further comprised of a common power level for all UEs in the cell (measured in dBm) and a UE-specific offset, and an open-loop path loss compensation component. The dynamic offset part of the power per resource block can also be further broken down into two components, a component dependent on the used MCS and explicit Transmitter Power Control (TPC) commands.
The MCS-dependent component (referred to in the LTE specifications as ΔTF, where TF stands for “Transport Format”) allows the transmitted power per RB to be adapted according to the transmitted information data rate.
The other component of the dynamic offset is the UE-specific TPC commands. These can operate in two different modes: accumulative TPC commands (available for PUSCH, PUCCH and SRS) and absolute TPC commands (available for PUSCH only). For the PUSCH, the switch between these two modes is configured semi-statically for each UE by RRC signalling—i.e., the mode cannot be changed dynamically. With the accumulative TPC commands, each TPC command signals a power step relative to the previous level.                Power Headroom Reporting        
In order to assist the eNodeB to schedule the uplink transmission resources to different UEs in an appropriate way, it is important that the UE can report its available power headroom to eNodeB.
The eNodeB can use the power headroom reports to determine how much more uplink bandwidth per subframe a UE is capable of using. This helps to avoid allocating uplink transmission resources to UEs which are unable to use them, in order to avoid a waste of resources.
The range of the power headroom report is from +40 to −23 dB. The negative part of the range enables the UE to signal to the eNodeB the extent to which it has received an UL grant which would require more transmission power than the UE has available. This would enable the eNodeB to reduce the size of a subsequent grant, thus freeing up transmission resources to allocate to other UEs.
A power headroom report can only be sent in subframes in which a UE has an UL grant. The report relates to the subframe in which it is sent. A number of criteria are defined to trigger a power headroom report. These include:                A significant change in estimated path loss since the last power headroom report,        More than a configured time has elapsed since the previous power headroom report,        More than a configured number of closed-loop TPC commands have been implemented by the UE.        
The eNodeB can configure parameters to control each of these triggers depending on the system loading and the requirements of its scheduling algorithm. To be more specific, RRC controls power headroom reporting by configuring the two timers periodicPHR-Timer and prohibitPHR-Timer, and by signalling dl-PathlossChange, which sets the change in measured downlink path loss to trigger a power headroom report.
The power headroom report is send as a MAC Control Element. It consists of a single octet where the two highest bits are reserved and the six lowest bits represent the dB values mentioned above in 1 dB steps. The structure of the MAC Control Element is shown in FIG. 7.
The UE power headroom PH valid for subframe i is defined by:PH (i)=PCMAX−{10 log10(MPUSCH(i))+PO_PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}[dB]
The power headroom shall be rounded to the closest value in the range [40; −23] dB with steps of 1 dB.
Pcmax, the maximum UE Transmission power (Tx power) is a value chosen by the UE in the given range of PCMAX_L and PCMAX_H.PCMAX_L≤PCMAX≤PCMAX_H, wherePCMAX_L=MIN{PEMAX−ΔTC, PPowerClass−MPR−A-MPR−ΔTC}, andPCMAX_H=MIN{PEMAX, PPowerClass};and where PEMAX is the value signalled by the network.
MPR is a power reduction value used to control the adjacent channel leakage power ratio (ACLR) associated with the various modulation schemes and the transmission bandwidth.
A-MPR is the additional maximum power reduction. It is band-specific and it is applied when configured by the network. Therefore, Pcmax is UE-implementation-specific and hence not known by eNB.
More detailed information with regard to ΔTC is specified in 3GPP TS 36.101, Vers. 12.0.0, section 6.2.5, incorporated herein by reference.                LTE Device-to-Device (D2D) Proximity Services        
Proximity-based applications and services represent an emerging social-technological trend. The identified areas include services related to commercial services and Public Safety that would be of interest to operators and users. The introduction of a Proximity Services (ProSe) capability in LTE would allow the 3GPP industry to serve this developing market, and will, at the same time, serve the urgent needs of several Public Safety communities that are jointly committed to LTE.
Device-to-Device (D2D) communication is a technology component for LTE-rel. 12. The Device-to-Device (D2D) communication technology allows D2D as an underlay to the cellular network to increase the spectral efficiency. For example, if the cellular network is LTE, all data carrying physical channels use SC-FDMA for D2D signalling. In D2D communication, user equipments (UEs) transmit data signals to each other over a direct link using the cellular resources instead of through the Base Station. A possible scenario in a D2D compatible communication system is shown in FIG. 9.                D2D Communication in LTE        
The “D2D communication in LTE” is focusing on two areas: Discovery and Communication, whereas this invention is mostly related to the Discovery part.
Device-to-Device (D2D) communication is a technology component for LTE-A. In D2D communication, UEs transmit data signals to each other over a direct link using the cellular resources instead of through the BS. D2D users communicate directly while remaining controlled under the BS, i.e., at least when being in coverage of an eNB. Therefore D2D can improve system performances by reusing cellular resources.
It is assumed that D2D operates in uplink LTE spectrum (in the case of FDD) or uplink subframes of the cell giving coverage (in case of TDD except when out of coverage). Furthermore D2D transmission/reception does not use full duplex on a given carrier. From individual UE perspective, on a given carrier D2D signal reception and LTE uplink transmission do not use full duplex, i.e., no simultaneous D2D signal reception and LTE UL transmission is possible.
In D2D communication when UE1 has a role of transmission (transmitting user equipment), UE1 sends data and UE2 (receiving user equipment) receives it. UE1 and UE2 can change their transmission and reception role. The transmission from UE1 can be received by one or more UEs like UE2.
With respect to the user plane protocols, in the following the content of the agreement from D2D communication perspective is reported (3GPP TR 36.843 vers. 12.0.0 section 9.2, incorporated herein by reference):                PDCP:                    1:M D2D broadcast communication data (i.e., IP packets) should be handled as the normal user-plane data.            Header-compression/decompression in PDCP is applicable for 1:M D2D broadcast communication.                            U-Mode is used for header compression in PDCP for D2D broadcast operation for public safety.                                                RLC:                    RLC UM is used for 1:M D2D broadcast communication.            Segmentation and Re-assembly is supported on L2 by RLC UM.            A receiving UE needs to maintain at least one RLC UM entity per transmitting peer UE.            An RLC UM receiver entity does not need to be configured prior to reception of the first RLC UM data unit.            So far no need has been identified for RLC AM or RLC TM for D2D communication for user plane data transmission.                        MAC:                    No HARQ feedback is assumed for 1:M D2D broadcast communication.            The receiving UE needs to know a source ID in order to identify the receiver RLC UM entity.            The MAC header comprises a L2 target ID which allows filtering out packets at MAC layer.            The L2 target ID may be a broadcast, group cast or unicast address.                            L2 Groupcast/Unicast: A L2 target ID carried in the MAC header would allow discarding a received RLC UM PDU even before delivering it to the RLC receiver entity.                L2 Broadcast: A receiving UE would process all received RLC PDUs from all transmitters and aim to re-assemble and deliver IP packets to upper layers.                                    MAC subheader contains LCIDs (to differentiate multiple logical channels).            At least Multiplexing/de-multiplexing, priority handling and padding are useful for D2D.                        Resource Allocation        
The resource allocation for D2D communication is under discussion and is described in its present form in 3GPP TR 36.843, version 12.0.0, section 9.2.3, incorporated herein by reference.
From the perspective of a transmitting UE, a UE can operate in two modes for resource allocation:                Mode 1: eNodeB or Release-10 relay node schedules the exact resources used by a UE to transmit direct data and direct control information;        Mode 2: a UE on its own selects resources from resource pools to transmit direct data and direct control information.        
D2D communication capable UE shall support at least Mode 1 for in-coverage. D2D communication capable UE shall support Mode 2 for at least edge-of-coverage and/or out-of-coverage.
UEs in-coverage and out-of-coverage need to be aware of a resource pool (time/frequency) for D2D communication reception.
All UEs (Mode 1 (“scheduled”) and Mode 2 (“autonomous”)) are provided with a resource pool (time and frequency) in which they attempt to receive scheduling assignments.
In Mode 1, a UE requests transmission resources from an eNodeB. The eNodeB schedules transmission resources for transmission of scheduling assignment(s) and data.                The UE sends a scheduling request (D-SR or RA) to the eNodeB followed by a BSR, based on which the eNodeB can determine that the UE intends to perform a D2D transmission as well as the required amount resources.        In Mode 1, the UE needs to be RRC Connected in order to transmit D2D communication.        
For Mode 2, UEs are provided with a resource pool (time and frequency) from which they choose resources for transmitting D2D communication.
FIG. 8 schematically illustrates the Overlay (LTE) and the Underlay (D2D) transmission and/or reception resources. The eNodeB controls whether the UE may apply Mode 1 or Mode 2 transmission. Once the UE knows its resources where it can transmit (or receive) D2D communication, it uses the corresponding resources only for the corresponding transmission/reception. In the example of FIG. 8, the D2D subframes will only be used to receive or transmit the D2D signals. Since the UE as a D2D device would operate in Half Duplex mode, it can either receive or transmit the D2D signals at any point of time. Similarly, in the same figure, the other subframes can be used for LTE (overlay) transmissions and/or reception.
D2D discovery is the procedure/process of identifying other D2D capable and interested devices in the vicinity. For this purpose, the D2D devices that want to be discovered would send some discovery signals (on certain network resources) and the receiving UE interested in the said discovery signal will come to know of such transmitting D2D devices. Ch. 8 of 3GPP TS 36.843 describes the available details of D2D Discovery mechanisms.                D2D Discovery        
ProSe (Proximity based Services) Direct Discovery is defined as the procedure used by the ProSe-enabled UE to discover other ProSe-enabled UE(s) in its proximity using E-UTRA direct radio signals via the PC5 interface. FIG. 10 schematically illustrates a PC5 interface for device-to-device direct discovery.
Upper layer handles authorization for announcement and monitoring of discovery information. For the purpose, UEs have to exchange predefined signals, referred to as discovery signals. By checking discovery signals periodically, a UE maintains a list of proximity UEs in order to establish communication link when it is needed. Discovery signals should be detected reliably, even in low Signal-to-Noise Ratio (SNR) environments. To allow discovery signals to be transmitted periodically, resources for Discovery signals should be assigned.
There are two types of ProSe Direct Discovery: open and restricted. Open is the case where there is no explicit permission that is needed from the UE being discovered, whereas restricted discovery only takes place with explicit permission from the UE that is being discovered.
ProSe Direct Discovery can be a standalone service enabler in a discovering UE, which enables the discovering UE to use information from a discovered UE for certain applications. As an example, the information transmitted in ProSe Direct Discovery may be “find a taxi nearby”, “find me a coffee shop”, “find me the nearest police station” and the like. Through ProSe Direct Discovery a discovery UE can retrieve needed information. Additionally, depending on the information obtained, ProSe Direct Discovery can be used for subsequent actions in the telecommunication system, such as, for example, initiating ProSe Direct Communication.                ProSe Direct Discovery Models        
ProSe Direct Discovery is based on several discovery models. The models for ProSe Direct Discovery are defined in 3GPP TS 23.303 V12.0.0, section 5.3.1.2 which is enclosed herein by reference: